Among the various machining operations which may be performed using machine tools it is possible to identify so-called “point” type machining operations, where the tool is positioned on a specific point of the surface to be machined often in a direction perpendicular thereto, but also, in particular circumstances, in a direction inclined relative thereto and is then moved along its axis so as to be able to perform machining operations such as boring, countersinking, counterboring or threading.
Another type of machining operation are machining operations of “surface machining” type, namely machining operations such as milling or leveling whereby flat or curved surfaces are formed. In these machining operations, the tool, once the working depth has been defined, carries out machining, if necessary in several passes to increasingly greater depths, moving in directions substantially perpendicular to its axis of rotation.
The present invention relates mainly to the first type of machining operation, but also to particular cases of the second type of machining operation.
Below a possible example of a sequence of operations for machining of the “axial” or “point” type carried out perpendicularly with respect to the surface will be briefly described.
Before carrying out the operation, the end of the tool is positioned in the vicinity of the point of the surface the workpiece (defined “positioning point”) with the machining head and therefore the spindle directed so that the axis of the tool is arranged along the perpendicular to the plane tangential to the surface passing through the contact point.
Then the end of the tool is positioned on the point of the surface of the workpiece (defined “contact point) where machining must be started.
As is known, numerical control machines comprise a programmable control unit and operate by means of a CAM program containing the machining cycle and other information necessary for machining the workpiece depending on its geometrical form.
In particular, owing to the control unit, the aforementioned machining operation may be performed in three steps:
1) a fast positioning step (also called “rapid movement”) where the machining head is moved so that the end of the tool is positioned on the positioning point, situated slightly above the contact point and with the axis aligned with the perpendicular to the surface of the workpiece;
2) a slow approach step where the tool spindle is moved forwards along the direction of its axis until the tip of the tool comes into contact with the surface of the workpiece, thus moving from the positioning point to the contact point;
3) a machining step where the tool is moved forwards along the direction of its axis and penetrates into the material by an amount equal to the machining depth in order to carry out the required machining.
In the case where machining of the “axial” type must be carried out with a given inclination relative to the perpendicular to the surface of the workpiece, the same working sequence is employed except that the axis of the head is suitably directed so that it is inclined by the desired amount relative to the perpendicular to the surface.
In the case instead of “surface machining” operations, such as milling, machining is composed of the following steps:
1) a fast positioning step where the machining head is moved so that the end of the tool is positioned on the positioning point, situated slightly above the contact point and with the axis aligned with the perpendicular to the surface of the workpiece;
2) a slow approach step where the spindle with tool is moved forwards along the direction of its axis until the end of the tool comes into contact with the surface of the workpiece and then penetrates into the material to be machined to a depth equal to the machining pass;
3) a third step where the machining head is moved in the direction perpendicular to its machining axis in order to carry out the required machining operation, such as machining of a groove or a pocket.
Whatever the machining operation to be performed, it is of the fundamental importance that the tool, before starting machining, should be positioned correctly relative to the workpiece to be machined.
After positioning of the workpiece on the table (to be performed with great care), once the workpiece has been fixed, its fixing is checked, and using suitable measuring instruments, its correctly position in the machine.
Although the machines of the prior art are widely used and popular, they are not without drawbacks.
During the first positioning step, it may happen that the spindle is not positioned in the desired manner for various reasons linked, for example, to the article being machined, e.g.:
1) non-correspondence between the real dimensions of the article being machined and the designed dimensions;
2) imprecise positioning of the article being machined on the workpiece table;
3) movement of the article owing to a faulty fixing system.
As regards the tool, for example imprecise positioning of the tool tip may occur, namely the tip of the tool is not located at the required positioning point, or the axis of the tool is not precisely directed, or the axis is not perfectly perpendicular namely it is not inclined at the desired angle relative to the perpendicular to the surface.
In these situations the machining operations carried out have a depth and/or a position different from that desired.
The prior art has attempted to solve this technical problem by using sensing feelers mounted on the spindle nose, consisting of a single finger provided with a transducer able to detect with a series of sensing operations the real form and position of the surface on which machining is to be performed. The feeler may consist of many types, for example: mechanical contact (by means of transverse deflection or axial pressure) type, but also electrical contact, optical or opto-electronic type.
It has been noted, however, that the time required for machining increases significantly. In fact it is required to remove firstly the tool from the spindle nose and replace it with the sensing device which must then perform sensing of the surface by means of a plurality of sensing operations on specific points. Once the surface has been sensed, a control unit compares the measured values with the theoretical surface values. If the control unit determines a difference between the two values, it calculates the divergence between the real configuration and the theoretical configuration. Once the divergence, if present, has been calculated and the sensing tool replaced with the machining tool, the control unit moves the spindle applying a positioning correction depending on the divergence determined.
It is clear that, during this procedure, in addition to the time needed for sensing the surface of the article by the device, there is also the time required both for the tool changing operation and for moving the spindle between the tool crib and the surface of the article to be machined.
It has also been noted that the device and the sensing method described above are not suitable for solving positioning problems associated with a possible deformation of the material during machining.
This may happen for example, during the machining of articles such as metal sheets, which are thin and therefore not very rigid, or in the case of composite articles such as those where there is a stiffening structure lined on both sides with a covering surface, such as the wing structure of aircraft. In the zones of the covering surface which are situated at a distance from the rigid structure, the material is more deformable such that, following the stresses caused by machining, it could also flex or bend with the result that the machining is not carried out in a precise manner.
In extreme cases breakage or structural failure of the article being machined could also occur.
Finally, the system employed by the prior art is also not effective in the case where the article has moved owing to improper fixing.
All this results in a further increase in the production costs since the article resulting from imprecise machining must be subsequently discarded.
The object of the invention is therefore to solve, at least partially, the drawbacks of the prior art.